transformer size

[daddy0]

have 3 ph 4wire 200amp 480/277v panel need 120 volt panel to feed 36 diesel heaters at 11 amps each. Came up with 100kva step down transformer feeding 2 200amp 120/208v panels. did i calculate correct?

 

[LarryFine]

Dad, welcome to the forum!

have 3 ph 4wire 200amp 480/277v panel need 120 volt panel to feed 36 diesel heaters at 11 amps each. Came up with 100kva step down transformer feeding 2 200amp 120/208v panels. did i calculate correct?

Let's see: 36 x 120 x 11 = 47,520. You're just under half of the transformer's capacity.

With 12 heaters per phase, your load would be 132a. You could use a single 200a panel.

 

[jumper]

I probably did something wrong. I came out with a 75kVa transformer and 1 200 amp 40 space 208/120 panel.

36x11x120x1.25=59,400VA
59,400Va/(208x1.732)=165 amps per leg

 

[jumper]

Dad, welcome to the forum!


Let's see: 36 x 120 x 11 = 47,520. You're just under half of the transformer's capacity.

With 12 heaters per phase, your load would be 132a. You could use a single 200a panel.

Larry don't I have to multiply those heaters by 1.25 also? or are t-stats involved here?

 

[LarryFine]

Larry don't I have to multiply those heaters by 1.25 also? or are t-stats involved here?

I presumed the latter, but you could be right.

Even so, I'd say our single-200a-panel idea is sound.

 

[iwire]

Larry don't I have to multiply those heaters by 1.25 also? or are t-stats involved here?

I think we could argue that forever.

At 47.5 KW I would likely just go with a 75 KVA transformer supplying a 225 amp panel.

 

[iwire]

Even so, I'd say our single-200a-panel idea is sound.

I agree.

But a very common installation in my area is a 75 trans supplying a 225 MB 3 phase panel. Readily available and allows the potential full use of the 208 amps the trans is rated for.

 

[erickench]

Larry don't I have to multiply those heaters by 1.25 also? or are t-stats involved here?

NEC 424.3(B) state's that fixed electric space heating shall be considered continuous load. These are diesel heaters. What are they used for? NEC 427.4 say's the same for fixed electric heating equipment for pipelines and vessels.

 

[iwire]

I do not think either of those sections applies to engine block heaters.

 

[erickench]

Then there's nothing in the NEC that state's that this load should be treated as continuous. What does the manufacturer say?

 

[kingpb]

You don't need to multiply the transformer by 1.25 The minimum transformer would be according to the first post, i.e. 47.52KVA. If you could show that the heaters will not be operating at the same time (running versus connected) then you might be able to prove a 45KVA is adequate. That is if you are trying to go cheapest first cost. Otherwise, if you bump up to 75KVA, then why not install full capability, as mentioned.

As far as the 1.25. The 1.25 factor should be applied to the equipment rating, not the load. The load is 47.52KVA, the load will never be 47.52KVA x 1.25. The breakers, cable, and panel need to be rated for 1.25 greater than the load, but not the transformer.